Your luck is better than mine. I've had a broken J seat on three out of my four HA flights. Every time it's been the same seat on the same aircraft (I was told that the parts for it are no longer made, so it's been running broken - and now it's two seats, not just one - for a while). But HA always called me proactively several hours before flight and offered to fly me on ANA A380 First class (a much better soft and hard product - full suite, not just a lie flat as HA J is) or provided other alternatives. And that was on another airline ticket award, not even revenue, tix.

DL, as always, is in last place when it comes to customer service (customers last is quite clearly the Delta M.O.). There is no excuse why in cases like the OP and others describe here, where the issue is well known, as soon as the aircraft operating on the route is known (i.e. some 6~12 hours prior to departure), for the departure airport station not to call the affected pax and provide them with a different seat or alternate flights on another (better) airline.

Until consistently deferring maintenance and cutting corners starts cutting into DL's bottom line and financials, and starts costing DL more than it is "saving," DL will keep doing this. Affected pax should always request full cash compensation for the J vs. Y fare difference, at the very least.